24 May 2018

Publication in PNAS

Plants make own defence against pathogenic fungi

Plants defend against pathogenic bacteria and fungi around their roots by secreting a protective substance. The beneficial soil bacteria Pseudomonas activates the gene in the plant that allows it to produce the substance. Researchers from Utrecht University published this finding last month in the prestigious scientific journal PNAS. Professor of Plant-Microbe Interactions Corné Pieterse: “Now we finally know how plants influence their microbiomes to tailor their environment to their needs.”

Plant growth and immune systems are strongly influenced by their ‘microbiome’, the enormous collection of bacteria and fungi in their surroundings. Scientists have long known that plants depend on the beneficial bacteria in their microbiome for their physiology - nutrient intake, immune systems, and the size of their root system. The probiotic bacteria Pseudomonas inhibits the growth of pathogenic fungi around the roots of the model plant Arabidopsis thaliana. But the researchers have now discovered that Pseudomonas not only fights the fungi directly, it also activates a gene in the roots that makes the plant create and secrete a substance that attacks the pathogenic fungi in the soil.

Scopoletin

Postdoc researcher Giannis Stringlis conducted the research, which earned the attention of the whole field of study dealing with the relationship between plants and microorganisms. Stringlis explains: “Pseudomonas stimulates the plant’s immune system, but only when the plant grows from a seed in the presence of the bacteria. That is when the plant makes the defensive substance scopoletin.”

Plants that do (left) and don't (right) secrete scopoletin with their roots.

UV light

The researchers discovered that the transcription factor gene MYB72 remains deactivated when Arabidopsis is not exposed to Pseudomonas during growth. In order to determine the influence of Pseudomonas, they intentionally deactivated the gene responsible for the secretion of scopoletin. Stringlis: “The resulting mutant plants produced little or no scopoletin, even in the presence of Pseudomonas. The results were easy to observe, because scopoletin is a coumarine, a group of fluorescent phenolic compounds.” Stringlis: “The roots of plants with scopoletin glow brightly under UV light, but the roots of the myb72 mutant did not.”

 

The circle is closed

Corné Pieterse, leader of the Plant-Microbe Interactions research group, sees a promising future for the line of research initiated with this publication. “Until now, we only knew that beneficial microorganisms could stimulate the plant’s growth and immune system. But now we know how a plant can influence the composition of its own microbiome. That closes the circle of influence between plants and microorganisms. This knowledge is valuable for sustainable agriculture, because it will allow us to use less pesticides and fertiliser to produce our food.”